Silver halide photography has been much more universally employed in the past, compared with electrophotography, diazo photography and the like, because of the superior photographic characteristics such as sensitivity, gradation, etc., of silver halide photography. However, silver halide photography requires much time and labor, because the silver halide light-sensitive material employed in this method must be subjected to several processings including an image-exposure, a developing process using a developer and process for preventing the developed image from changing color or deteriorating under normal room-illumination and preventing the non-developed portion (hereinafter background) from blackening, e.g., processing including stop, fixation, washing and rinsing, stabilizing and other similar processes. In addition, the chemical agents which may be used in this method are dangerous to the human body and the processing room and the workers' hands and clothes are often stained with these agents. Therefore, it has been strongly desired to improve silver halide photography so that the light-sensitive materials can be treated in a dry condition instead of treatment with solutions, and so that the processed images are maintained stable. In order to solve this problem, many efforts have been made.
A first method which has been developed thus far includes the so-called combined developing and fixing bath method wherein two procedures in a conventional silver halide photography, developing and fixing procedures, can be replaced by one procedure, as disclosed in U.S. Pat. No. 2,875,048; British Patent No. 954,453; and German Patent Application OLS No. 1,163,142. A second method attempts to replace wet procedures in conventional silver halide photography with dry procedures, as disclosed in German Patent Application OLS No. 1,174,159; British Patent Nos. 943,476 and 951,644; and so on. A third method uses as a main light-sensitive component a silver salt of a long chain aliphatic carboxylic acid such as silver behenate, silver saccharin or silver benzotriazole, etc. and a catalytic amount of a silver halide simultaneously, as disclosed in U.S. Pat. Nos. 3,152,904; 3,457,075; 3,635,719; 3,645,739; and 3,756,829 and Canadian Patent No. 811,677; and so on.
However, the unexposed parts of the heat-developed light-sensitive materials which have so far been proposed, for example, the unexposed parts of the compositions containing the silver salts of fatty acids such as silver behenate, etc., reducing agents and catalytic amounts of silver halides become to a considerable extent black. It makes the distinction between the images and the background difficult because there is very little contrast between the black images formed on the exposed parts by heating (image density) and the fogged black background. Therefore, a reduction of fog has been an important subject in this art. Moreover, storage of light-sensitive materials for a long time before use under conditions of high temperature (30.degree. C.-50.degree. C.) and high humidity (more than 50% relative humidity) causes fog resulting in the formation of indistinguishable images.
A particular problem with dry laser films containing a silver behenate melt is fog, such as pepper fog, which may appear as black spots in unexposed areas on film such as microfilm.
U.S. Pat. No. 3,871,887 describes a photothermographic composition containing a halide salt to increase the photosensitivity of the photothermographic composition.
U.S. Pat. No. 4,273,723 by Hayashi et al. describes high purity silver salts of organic carboxylic acids. Column 5, lines 54 to 59 clarifies purity to the silver content of the silver behenate. This would mean that purity refers to conversion of the free acid to the silver salt. There is no measurement of the purity of the organic carboxylic acid.
In U.S. Pat. Nos. 5,443,742 and 5,512,185 the removal of reducing impurities from behenic acid and other organics by treating with AgO, and other oxidizing agents (MnO.sub.2, PbO) is discussed. Analytical measurement of reducing impurities is implied to check levels of reducing impurities. No method is mentioned nor are any levels of reducing impurities given. The indications are that this standard test does not determine or is insensitive to the actual impurities present. These patents are based on a method to remove unspecified materials with no definition of what or how much is being removed.
In U.S. Pat. No. 3,997,597 the process of making Ag salts in the presence of Hg and Pb salts is described. It is proposed to affect particle size, which is tied to thermal fog, density and contrast. No tie to reducing impurities or purity of the carboxylic acid is mentioned in U.S. Pat. No. 3,960,908. Fog is related to residual alkali content.
Silver behenate and other fatty acids are used in many dry photothermographic and thermographic processes. The starting material, fatty acids from natural sources, is purchased in large lots and purified before use because the crude material has been found to cause fogging. The purification process however is quite expensive.
Many materials in a photothermographic and thermographic composition are accompanied by serious fog production. Under these circumstances, further improvement is required with respect to said photothermographic and thermographic materials.